This invention generally relates to a microstrip antenna device for circularly polarized waves.
A microstrip antenna comprises a dielectric sheet with a conductor mounted on one surface and a ground conductor mounted on the other surface. Such an antenna utilizes the radiation loss of an open planar resonance circuit. Attention is now being focused on such microstrip antennas because of their low profile, reduced weight, compactness and ease of manufacture.
FIG. 7 shows one form of a conventional microstrip antenna device for circularly polarized waves. As shown, the device comprises a dielectric sheet 110 which is sufficiently thin with respect to the wavelength used. One surface of dielectric sheet 110 has a radiating conductor sheet 120 formed from a copper foil while the other surface is entirely covered with a ground conductor sheet 130 also formed from a copper foil. This arrangement defines a microstrip antenna device. The device further includes a feeder in which a small hole 111 is formed that extends through the dielectric sheet 110, the radiating conductor sheet 120 and the ground conductor sheet 130. A connector 140, or more precisely, an external conductor associated therewith, is soldered to the ground conductor sheet 130. The internal conductor or core of the connector 140 is connected to a gold plated wire 141 which is soldered to the feeder portion of the radiating conductor sheet 120. The hole 111 is filled with an insulating material, not shown, which insulates the wire 141 from the ground conductor sheet 130. The connector 140 is coupled to a coaxial cable 150 which is in turn connected to the high frequency amplifier of a receiver unit, as indicated by an arrow designated R.F.Amp.
In the above-described microstrip antenna device for circularly polarized waves, the size of the radiating conductor sheet 120 is determined by the wavelength involved. No definite figure is given for the size of the ground conductor sheet 130, although it should theoretically be infinitely extensive in order to eliminate fringing effects. However, an infinitely extensive sheet is impractical and it has been the prior art practice that a sheet 130 of a size sufficiently larger in comparison to the size of the radiating conductor sheet 120 may be used. For example, sheet 130 may have one side which is three to four times as long as that of the radiating conductor sheet which is used to define a microstrip antenna device. When the radiating pattern which is actually generated is close to an ideal pattern and the axial ratio which is actually produced may be considered as representing a circularly polarized wave, it is concluded that the antenna device is adapted for practical use.
FIG. 4 shows an ideal radiating pattern for a microstrip antenna device for circularly polarized waves. The ideal pattern is indicated by the solid curve having a half-value angle .theta..sub.0 of about 75.degree. where a 3 dB reduction occurs and a lateral depression of about 14 dB.
Because of the low profile, light weight and compactness of the microstrip antenna, it is frequently mounted in a restricted space. In such applications, it is desirable that the antenna be as small as possible so long as it provides a comparable characteristic. However, such a requirement cannot be met with the conventional microstrip antenna. There have been no teachings in the prior art which permit this goal to be attained, despite the advantages associated therewith.